The present invention relates to an intermediate transfer sheet. More particularly, the invention relates to an intermediate transfer sheet for use in an intermediate transfer type thermal transfer recording method wherein an ink image is first formed on the intermediate transfer sheet by a thermal transfer method using a thermal transfer sheet and the ink image on the intermediate transfer sheet is then transferred onto a final image receptor including resin shaped articles such as cards (e.g. identification card), CD-Rs, and plates by a thermal transfer method using a thermal transfer printer equipped with a thermal head, a hot roller, or a hot press. In the present specification, the transfer of the ink image on the intermediate transfer sheet onto the final image receptor is sometimes referred to as “retransfer”.
Images formed on identification cards (ID cards) such as credit cards and membership cards by a melt-transfer type thermal transfer method are frequently required to have high level durability. In the conventional intermediate transfer type thermal transfer recording method, there has been adopted a method wherein an intermediate transfer sheet having a protective layer is used to improve the durability of the image on the final receptor (see patent reference 1). Thermoplastic resins having good durability such as acrylic resins are used in the protective layer. When a durability better than that of the thermoplastic resins is required, a method wherein the protective layer is composed of a thermosetting resin is used. Generally the curing of the protective layer is conducted by using heat or ionizing radiation. In the case of curing with heat, the protective layer is previously cured in a state wherein it is present in an intermediate transfer sheet because the heat curing usually needs a long period time and the heat curing of the protective layer after being transferred results in degradation in easiness of the system. Further the heat curing after the transfer frequently causes drawbacks such as heat deformation of image bearing articles.
In the case of curing using ionizing radiation, the protective layer may be cured either before or after the transfer from the viewpoint of easiness of the system. However, when the protective layer is previously cured in a state wherein it is present in the intermediate transfer sheet to improve the durability as described above, the heat-meltability of the protective layer is markedly degraded resulting in poor selective transferability of the protective layer. The term “selective transferability”, as used herein, refers to the property that only a heated portion of a transfer layer is transferred but unheated portions in the periphery of the heated portion are not transferred. As a result, in the transfer of the protective layer at the edge portion of character, there sometimes occurs a phenomenon that a heated portion of the protective layer is transferred together with an unheated portion adjacent to the heated portion (this phenomenon is referred to as “planar peeling”). In this case, it is necessary to make the protective layer thinner to obtain a satisfactory selective transferability and the thickness of the protective layer is limited to a relatively smaller one, resulting in failure to achieve high level durability.
In the case of curing the protective layer by irradiation with an ionizing radiation after the transfer, there are no such problems as mentioned above but there is another problem as follows: Generally ionizing radiation-curable resins have tackiness before curing. It is difficult to form a receptive/adhesive layer, which is provided to print a good image on the intermediate transfer sheet, onto the protective layer due to the tackiness of the protective layer, resulting in failure to obtain a satisfactory intermediate transfer sheet.
Patent reference 1: JP A 10-297122
It is an object of the present invention to provide an intermediate transfer sheet on which a satisfactory ink image is formed by a thermal transfer method using a thermal transfer sheet and which provides an image having excellent durability on a final receptor.
This and other objects of the present invention will become apparent from the description hereinafter.